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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0837Z Dec 12, 2018)
 
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Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Wed Dec 12 2018

Valid 12Z Wed Dec 12 2018 - 12Z Sat Dec 15 2018

...Pacific Northwest through the Central Rockies...
Days 1-3...

Two distinct shortwaves, the first today, and the second on
Friday, will bring heavy mountain snows to the Mountain West this
period. The first impulse will push inland from the Washington
coast today along with a surface cold front and a plume of
significant moisture. Westerly 850-700mb wind combined with jet
level diffluence will drive ascent into the Washington Cascades,
with subsequent moisture spillover into the other west mountain
ranges being squeezed out as snow through height falls and
easterly advecting jet diffluence. WPC probabilities are high for
8 inches of snow in the Washington Cascades, with high
probabilities for 4 inches in the Bitterroots, Tetons, and
Colorado Rockies. The highest snow totals will be confined above
6000 feet.

After a brief lull, the second significant shortwave will move
into Washington state on Friday. Another push of Pacific moisture
will saturate the column across Washington and Oregon, with
pronounced lift within the Pacific jet producing snow across the
Cascades of Oregon and Washington, as well as the northern Rockies
into Montana. The trough axis will not move onshore until late on
day 3, so warm advection ahead of this feature will raise snow
levels back to 5-6 kft before crashing Friday night into Saturday.
This will keep the heaviest snow above these levels, and WPC
probabilities show a high risk for 4 inches of snow only in the
highest terrain of the Cascades.


...Southern Plains...
Day 2...

A northern stream shortwave trough will dig from the Pacific
Northwest while deepening into the Southern Plains Thursday
morning. This feature is likely to close off as it becomes
anomalously deep, as evidenced by forecast 500mb heights of -4
standard deviations below climo. Beneath this feature, a surface
low pressure will move east across Texas, with the system becoming
vertically stacked late on Day 2. The guidance continues to insist
that an area of heavy snow will develop west of the upper low, but
timing and placement details persist a lower than average
confidence forecast.

Although column temperatures will be marginal, rain will likely
transition to snow as an area of 850-600mb frontogenesis
coincident with a well defined TROWAL produces strong ascent,
aided by theta-e lapse rates dropping below 0 in the vicinity of
the saturated DGZ. Despite cold advection at 850mb, warm advection
within the TROWAL wrapping cyclonically around the upper low will
limit lapse rate steepening, important since dynamic cooling due
to strong snowfall rates will be required to produce significant
accumulations. Still, there is good model consensus among the
NAM/GFS/ECMWF and their ensembles that more than 4 inches of snow
will occur in a north-south oriented stripe across north-central
Texas. WPC probabilities echo this despite likely low SLRs, and a
few places may exceed 6 inches if the frontogenetical band becomes
intense enough.


The probability of significant icing (amounts of 0.25 inch or
more) is less than 10 percent. 


Weiss